The efficiency for collimated positronium production by charge-exchange in positron collisions with gaseous targets has been investigated in the range 20-396 eV. At 250 and 396 eV, CO2 has been found to be approximately twice as efficient as N-2, the previous best neutralising gas at high energies. The efficiency from Xe, whilst lower at low energies, becomes comparable to that from H-2 at around 100-120 eV; at similar to 250 eV, it is an order of magnitude lower.